1. Field of the Invention
The present invention relates to a shadow mask which is applicable to forming a film having predetermined patterns. The present invention also relates to a method of forming the shadow mask. The present invention further relates to a method of manufacturing a semiconductor device with using the shadow mask.
2. Description of the Related Art
An organic EL (Electroluminescence) display, which is one of semiconductor devices, has been employed in a television, a computer, and the like.
In order to realize color display, a plurality of light emission layers are selectively formed in predetermined areas in the organic EL display device. Each of the light emission layers emits light of red, green or blue. The formation of the light emission layers requires a metal sheet having apertures as a mask (shadow mask). The light emission layers are formed by evaporation.
For example, techniques disclosed in Unexamined Japanese Patent Application KOKAI Publications Nos. H3-250583 and H8-227276 relate to a method of manufacturing the organic EL display device.
FIG. 7 is a cross sectional view showing a state of forming the light emission layers with using a shadow mask described above.
A transparent anode 502 is formed on a transparent glass substrate 501, and a hole transfer layer 503 is formed on the anode 502. As shown in FIG. 7, a shadow mask 504 is arranged so as to be slightly distant from the hole transfer layer 503. An evaporation material 505 is placed so as to face the hole transfer layer 503 so that the shadow mask 504 intervenes between them.
The evaporation material 505 is heated, thus light emission layers 506 are formed on predetermined regions of the hole transfer layer 503. Thereafter, an electron transfer layer and a cathode (not shown) are formed on the hole transfer layer 503 and the light emission layers 506, thus the organic EL display device is completed.
In such the case using the shadow mask, a thinner shadow mask with finer apertures is required to form finer light emission layers. However, thinner the shadow mask becomes, harder to realize the fine apertures, because the shadow mask is made of the metal sheet. The thin shadow mask may be bent by heat for evaporation. Accordingly, it is difficult to form the light emission layers with precise dimensions.
Moreover, the light emission layers or the like may be broken when the shadow mask contact it, because of the shadow mask is made of metal. In such a case, yield of the organic EL display device may be deteriorated.
Unexamined Japanese Patent Application KOKAI Publications Nos. S53-75858, S62-297457 and H4-236758, and Unexamined Japanese Utility Model Application KOKAI Publication No. S6445160 disclose masks for solving the above problems.
Unexamined Japanese Patent Application KOKAI Publication No. S53-75858 discloses a mask (evaporation mask) which comprises a thin silicon plate including boron and a support plate for reinforcing the silicon plate so as to have enough mechanical strength. Those thin silicon plate and support plate are formed unitarily. The thin silicon plate has a first hole whose side wall is perpendicular to a major surface of the thin silicon plate. The support plate have a second hole whose side wall is perpendicular to a major surface of the support plate. The second hole is larger than the first hole. In a case of forming a film on a substrate by evaporation, the mask is arranged so that its thin silicon plate faces the substrate.
In the case of forming the film on the substrate by the evaporation, some of evaporated particles slant toward the substrate. Since the side wall of the hole in the mask is perpendicular to the major surface of the mask (or the substrate) as described above, edge of the hole in the mask prevents the evaporated particles slanting toward the substrate from reaching the substrate. As a result, the formed film does not always have correct dimensions.
The mask and the substrate may be expanded by temperature variation or the like during the film formation. If the material of the substrate differs from that of the thin silicon plate of the mask, the thermal expansion rate of the substrate also differs from that of the thin silicon plate. In a case where, for example, the thin silicon plate includes boron as described above, such the problem may occur when the material of the substrate is not silicon including boron. As a result, the dimensions of the formed film may differ from the planned value. Especially in the semiconductor device whose pattern is very fine, minute difference may cause short circuit or the like, and yield of the semiconductor device may be deteriorated.
Unexamined Japanese Patent Application KOKAI Publication No. S62-297457 discloses a mask (mask for evaporation) has the thickness of 50 to 400 micrometers and a hole whose side wall is perpendicular to a major surface of the mask.
Such the mask is too thick to manufacture an organic EL color display device or the like. Since the side wall of the hole in the mask is perpendicular to the major surface of the mask, the mask prevents the evaporated particles slanting toward the substrate from reaching the substrate. Accordingly, a film having inappropriate dimensions may be formed.
Moreover, it is difficult to precisely form fine apertures necessary for forming a film having fine patterns even if anisotropic etching is carried out to form the apertures.
Unexamined Japanese Utility Model Application KOKAI Publication No. S64-45160 discloses a mask (mask for forming a film) which comprises stacked first and second masks. The first mask, which is relatively thin, has an opening whose shape is the same as that of a film to be formed, and the second mask, which is relatively thick, has an opening which is larger than the opening in the first mask. Since those first and second masks are stacked with each other, they are formed separately. Therefore, the first mask may slide away from the second mask and vice versa, or the thinner first mask may be bent because of thermal expansion caused by temperature variation or the like during the film formation.
Moreover, it is difficult to form a hole (an opening) in a very thin film because such a film does not have enough strength. Therefore, the film must have appropriate thickness (for example, 100 micrometers). Therefore, the first mask is made so as to be thinner than the second mask, however, the first mask is not very thin (approximately 10 micrometers, for example). As a result, edge of the hole in the mask prevents the evaporated particulars slanting toward the substrate from reaching the substrate, thus, it is difficult to form a film having appropriate dimensions.
Unexamined Japanese Patent Application KOKAI Publication No. H4-236758 discloses a mask (mask for evaporation) which is made of a (100) surface single crystal silicon wafer. Firstly, a region including an area which will mask apertures are applied (to-be-aperture area) of the (100) surface single crystal silicon wafer is processed so as to be thinner than surrounding regions. Then an SiO2 film is formed on regions other than the to-be-aperture area. Then apertures each of whose side wall is a (111) surface are formed by anisotropic etching the to-be-aperture area which is not covered with the SiO2.
Since there is an interrelation between the dimension of the aperture whose side wall is the (111) surface and the thickness of the mask, the above described mask is not suitable for forming a film having fine patterns.
Moreover, since the region including the to-be-aperture area is thinned first and then the apertures are formed as described above, the etching process is required twice. However, it is difficult to complete the first etching so that the single crystal silicon wafer has a predetermined thickness without failure, therefore, the to-be-aperture area may be too thin or too thick. As a result, the strength of the mask may decrease or apertures having appropriate dimensions are hardly to be formed.
Accordingly, if the masks disclosed in Unexamined Japanese Patent Application KOKAI Publications Nos. S53-75858, S62-297457 and H4-236758, and unexamined Japanese Utility Model Application KOKAI Publication No. S64-45160 are applied to manufacturing the organic EL display device, it is difficult to form the light emission layers each having appropriate dimensions. This causes deterioration of yield of the organic EL display device (semiconductor device).
It is an object of the present invention to provide a shadow mask by which a film having patterns of precise dimension is available. It is another object of the present invention to provide a method of forming a thin and strong shadow mask. It is a further object of the present invention to provide a method of manufacturing a semiconductor device with using the shadow mask by which a semiconductor device having excellent yield.
To accomplish the above objects, a shadow mask according to a first aspect of the present invention comprises:
a support layer;
a stopper layer, whose etching rate differs from etching rate of the support layer, formed on the support layer; and
a surface layer formed on the stopper layer,
wherein the support layer, the stopper layer and the surface layer comprise an opening having:
a tapered portion which is formed through the support layer, the stopper layer and the surface layer and is broadened toward the support layer from the surface layer; and
a projected portion which is a part of the surface layer projecting toward the center of the tapered portion.
According to this invention, the stopper layer is formed between the support layer and the surface layer. This structure allows that the support layer and the surface layer are processed separately while forming the opening by etching. Therefore, processing of the surface layer with excelent accuracy can be realized by etching.
A side of the projected portion may be broadened toward the support layer from. the surface layer.
The stopper layer may be a part of the support layer; and the stopper layer and the surface layer may be bonded together.
A method of forming a shadow mask according to a second aspect of the present invention comprises:
preparing a support layer having enough mechanical strength necessary for forming a predetermined hole therein;
forming on the support layer a stopper layer to be an etching stopper while forming the hole in the support layer;
forming a surface layer on the stopper layer;
etching a predetermined area of the support layer until the stopper layer is exposed to form the hole;
removing the stopper layer exposed through bottom of the hole;
etching the surface layer exposed through the bottom of the hole so as to have a predetermined thickness to form a tapered portion which is broadened toward the support layer from the surface layer; and
forming at the bottom of the hole a through hole whose diameter is smaller than that of the bottom of the hole to form an opening which comprises the tapered portion and a projected portion which is a part of the surface layer projecting toward the center of the tapered portion.
The support layer and the surface layer may be made of silicon; the stopper layer may be made of silicon dioxide; the etching the support layer may comprise etching of the support layer with a mixture gas of Cl2 and O2; the removing the stopper layer may comprise etching of the stopper layer with a Cl2 gas; and the etching the surface layer may comprise etching of the surface layer with a mixture gas of Cl2 and O2.
The etching the support layer, the removing the stopper layer and the forming the tapered portion may comprise selecting a layer to be etched by controlling the partial pressure of O2 in the mixture gas including Cl2 and O2.
The forming the stopper layer may comprise forming the stopper layer by thermal oxidizing a surface of the support layer; and the forming the surface layer may comprise bonding the surface layer to the stopper layer.
The forming the surface layer may comprise bonding the surface layer to the stopper layer with polyimide.
The forming the through hole may comprise forming the through hole by irradiating a laser beam onto the bottom of the hole.
A method of manufacturing a semiconductor device according to a third aspect of the present invention comprises:
preparing a substrate; and
forming a film having patterns regulated by predetermined dimensions on the substrate,
wherein the forming the film comprises forming the film with using a shadow mask which comprises:
a support layer;
a stopper layer, whose etching rate differs from etching rate of the support layer, formed on the support layer; and
a surface layer formed on the support layer,
wherein the support layer, the stopper layer, and the surface layer comprise an opening having:
a tapered portion which is formed through the support layer, the stopper layer and the surface layer and is broadened toward the support layer from the surface layer; and
a projected portion which is a part of the surface layer projecting toward the center of the tapered portion.
The surface layer may be made of a material which is the same as a material of the substrate or a material whose thermal expansion rate is substantially the same as thermal expansion rate of the substrate; and the forming the film may comprise arranging the shadow mask so that the surface layer faces the substrate.